Electronic signal tracking circuits



May 19, 1953 F. c. WILLIAMS E1' AL ELECTRONIC SIGNAL TRACKING CIRCUITS'7 Sheefcs-Sheet l Filed Aug. 6, 194'? Inventors 1?. c. williams dy N155May 19, 1953 F. c. WILLIAMS x-:r AL 2,639,419

ELECTRONIC SIGNAL TRACKING CIRCUITSY Filed Aug. 6, 1947 7 Sheets-Sheet 2May 19, 1953 F. c. WILLIAMS Er Al. 2,639,419

ELECTRONIC SIGNAL TRACKING CIRCUITS Filed Aug.' 6, 1947 7 Sheets-Sheet 3lnvenlor; F. G. '1111MB N. F. Hoody "es" 1M May 19, 1953 F. c. WILLIAMSE1- AL 2,639,419

ELECTRONIC SIGNAL TRACKING CIRCUITS v'7 Sheets-Sheet 4 loo K..

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May 19, 1953 F. C. WILLIAMS ET tAL ELECTRONIC SIGNAL TRACKING CIRCUITSFiled Aug. 6, 1947 7 Sheets-Sheet 5 6 MILES N :IMILE msecs.

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` [nvcnfors t. c 'mms May 19, 1953 F.` c. WILLIAMS ET AL ELECTRONICSIGNAL TRACKING CIRCUITS 7 sheets-sheet 'e Filed Aug. 6, 1947 00N n: on.2. oo. 2 om m ON n. O m

May 19, 1953 F. C. WILLIAMS El' AL ELECTRONIC SIGNAL TRACKING CIRCUITSFiled Aug. 6, 1947 Fig] Sheets-Sheet 7 Bounce) SWI i-sov.

Y j4OOV. lnvenlars l'. c. Inline En ya Urlley Patented May 19, 1953UITED STATES l PATENT GFFE'CE ELECTRGNIC SIGNAL TRACKING CIRCUITSApplication August 6, 1947, Serial No. 766,652 In Great Britain June 22,`1945 14 Claims.

The present invention relates to circuit arrangements for providing acontrol voltage when the relative timing of a recurrent input signal anda locally generated recurrent signal is such that the two signalsco-exist for at least a part of their duration.

The object of the invention is to provide circuit arrangements of theabove type which are more reliable in operation than previousarrangements and, in particular, which are not substantially affected bya reduction in the amplitude or strength of the input signal such as mayoccur, for instance, where radio signals, which are liable to fading,are involved.

According to the invention, a varying control voltage proportional tothe degree of coexistence of the two signals is developed and, `if theamplitude of the input signal falls below a minimum level necessary forthe generation of the control voltage, a local circuit is renderedeffective to provide a further control voltage equal or approximatelyequal to that `.which obtained prior to the input signal falling `belowsaid minimum level.

"The invention has application to a number (of diierent arrangements.

For instance, the timing of the locally generated .signal may be fixedWhile that of the input signal is variable. Alternatively the timing ofboth the locally generated signal and the input signal `may be variable.

Further the control voltage may be employed Afor a `number of diiierentpurposes. For instance, it may be employed for causing an indication ofthe input `signal to be given on a cathode ray tube. The input signalmay exist ,alone or it maybe one of a number oi co-exist- Aing signalsand, if suitable means vare provided for selecting a particular signal,the arrangement enables the variation in timing of a particular yone ofa'number of recurrent input signals to -be examined. Alternatively or inaddition the `control `voltage may be employed for maintaining thecol-existence of the input signal and the locally generated signal, so'that a recurrent indication is given of the variation in timing of theinput signal. Finally the control voltage may, when it attains Ia,predetermined value; cause the transmission of a further signal or setan additional circuit in operation.

The invention has particular application to `radio-location equipment inwhich the input signal lis produced by the reflection of a high`frequency `pulse-modulated carrier wave by .a distant object, such asan aircraft, and 'an indicaf iii) tion of the reflected or echo signalis displayed on the screen of a cathode ray tube. Usually a lineartime-base, synchronised with the transmitted pulse, is displayed on thescreen of the cathode ray tube and, when the input or echo signal andthe locally generated signal co-exist or overlap, the echo signal isdisplayed as a modification of the time-base trace to provide the rangeof the aircraft.

It should be pointed out that where the term echo signal is employed itshould be understood lto include a signal radiated by a transmitter inthe aircraft in `response to the reception in the aircraft of thepulse-modulated carrier Wave.

In one arrangement to which the invention may be applied, the timing ofthe locally generated signal is automatically varied progressively untilthe locally generated signal and the echo signal overlap when thecontrol voltage is generated and an indication is provided on the screenof a cathode ray tube. The subsequent timing of the locally generatedsignal is controlled by the control voltage so that a recur- `rentindication of the echo signal is obtained.

Alternatively the timing of the locally generated signal may be adjustedmanually until it overlaps the echo signal, whereupon, eitherVautomatically or by the operation of a switch or the like, thesubsequent timing of the local signal is effected by the control voltageso that the overlap between the two signals is maintained.

In a further arrangement to which the invention maybe applied, thetiming of the locally generated signal is manually adjusted to apredetermined value fand a control voltage is developed when the timingof the echo signal is such that the two signals overlap. The controlvoltage is employed for providing an indication of the echo signal andalso for modulating the transmitted carrier wave to enable the aircraftto follow a predetermined course, i. e. for the purpose of maintainingthe overlap of the two signals. The timing of the locally generatedsignal is adjusted so that it corresponds to the range of the target andthe signals transmitted to the aircraft enable it to follow a coursewhich passes over the target i. e. the overlap between the locallygenerated signal and the echo signal is maintained.

In a further arrangement, the timing of the locally generated signal isnot fixed but is automatically Varied until the two signals overlap, asecond locally generated signal being employed, the timing of which isadjusted to a xed value representing the target range. The circuit inthis case is so arranged that when the echo signal and the locallygenerated signal overlap, the timing of the locally generated signal iseffected by the control voltage and at the same time the carrier wavetransmitted to the aircraft is modulated to enable the aircraft tofollow a course such that the timing of the echo signal and the locallygenerated signal approach that of the second locally generated signal i.e. the aircraft follows a course which passes over the target.

The invention may also be applied to radiolocation equipment employingthe principle of automatic following. For instance the equipment may bemounted on an aircraft for detecting other aircraft for interceptionpurposes. The transmitting and receiving polar diagram of the equipmenttakes the form of a beam and arrangements are provided for causing thebeam to scan a certain field of View. When an echo signal is received,the regular scanning movement is terminated and the beam is caused Atobe lcontinually incident on the echo-reflecting aircraft substantiallyirrespective of the movement of both aircraft. Such irregular movementis controlled by a voltage dependent upon the degree of overlap betweenthe echo signal and a locally generated signal.

It will be seen that all these arrangements, which involve some form offollowing depend for their success upon the development of a, controlvoltage due to the overlapping of the echo signal and the locallygenerated signal. In the absence of the present invention this controlvoltage will only be developed if the amplitude of the echo signal isabove a certain minimum value. If the echo signal disappears entirely,for instance, due to fading, the control voltage will likewise disappearand the timing of the locally generatedsignal will, for example, revertto rits original progressive variation or will remain at the value whichobtained when the echo signal disappeared. The locally generated signalmay be said to be locked to the echo signal when the two overlap andsubsequently to follow the echo signal. If the echo signal fades, thelocally generated signal will become unlocked and will not again lock tothe echo signal until some adjustment has been made when the fade hasended. The examination of the echo signal is thus not possible duringthis period. When the present invention is applied to such arrangements,the constant control voltage which is locally generated is madeavailable and, unless the fade is of very long duration, increases thepossibility of the locally generated signal being still locked to theecho signal when the latter reappears.

According to a feature of the invention, the circuit arrangementscomprise means for generating a local signal consisting of twosubstantially square waveforms having coincident front or trailing edgesand different widths, means for obtaining a current proportional to thedegree of co-existence of the input signal with one of said Vsquarewaveforms and for obtaining a second equivalent to the mean value of thecontrolvolt- 4 age and if the amplitude of the input signal falls belowa predetermined minimum value, the condenser is arranged to dischargethrough the load so that a substantially constant control voltage -equalto the mean value is obtained. The condenser may be connected in acircuit which includes contacts of a relay, the operation of which iscontrolled in accordance with the strength of the incoming signal.

The invention will be better understood from the following descriptionof one embodiment as applied to a system of radio navigation taken inconjunction with the accompanying drawings in which:

Fig. 1 shows a block schematic of the equipment,

Fig. 2 shows part of the circuit of the magnified time base generator,

Figs. 3 and 4 show the circuit of the walking strobe unit,

Fig. 5 shows certain waveforms developed in the walking strobe unit, i.i

Fig. 6 shows diagrammatically the signals indicated on the long andmagnified time bases,

Fig. 7 shows the waveforms of the walking strobe and Fig. 8 shows thecircuit of the puise width control unit. v

Briefly, the radio navigational system to which the invention is appliedconsists of a ground station which radiates to an aircraft signals whichare modulated in accordance with the instantaneous distance between theground station and the aircraft and in accordance with the rate ofchange of said distance. The modulation of said signals is determined byinformation return by the aircraft in the form of echo or counterpartsignals and the reception of the modulated signals in the aircraftenables a predetermined path to be followed. In previous arrangements ofthis nature, the predetermined path was a circle with the ground stationat the centre and the signals transmitted by the ground station weremodulated in accordance with the distance of the aircraft from thepredetermined path, a distance known as the residual range of theyaircraft. It has been found that a better approach is obtained if thetransmitted signals are modulated not only in accordance with theresidual range but also with the rate of change of the residual range.The predetermined path is then exponential with regard to time and isasymptotic to the circular path. When employed for the purpose of blindbombing, the predetermined path is arranged to pass over the target anda second ground station is provided for transmitting a bomb-releasesignal at the appropriate instant. The first-mentioned ground station isreferred to as station A or the tracking station while thesecond-mentioned ground station is referred to as Station B. Bothstations are capable of operating either as an A or B station but thepresent invention is primarily of value for station A operation and thedescription will be confined to the operation of such station.

Two cathode ray tubes are provided at each station, one of whichdisplays a magnified portion of the time-base trace of the second. Thefirst of said tubes is termed the magnified time-base tube while thesecond is termed the long timebase tube. The time-base traces are linearand horizontal and are calibrated to give residual range of theaircraft. Two pulses are displayed on the magnified time-base trace, oneof which is known as the target marker pulse and means Lare "providedfor accurately setting vthe vlpulse to .fapositioncn the tune-base trace`representative of range-oi' the target. The second pulse'is `as the"walking strobe pulse the timing Ici' -vvhich maybe automatically-varied Yaccording to fthe of overlap wi-ththe echo ysignal 'so remainslocked to 'the echo signal. Voltages are alsoideveloped which areproportional to the residual `range `of the aircraft and :also fto `thevrate `of change 'of said range. These two voltiages aretedto arpulsewidthcontrol unit iior controllingrtlre Width of the transmitted pulseswhich thereby Sform a communication ychannel between "the ground stationand' tbefaircraft.

n The equipment will rst be described with refin IFlg. 1. The operationof the equipment is under ithe .control :of .a calibrator unit `CAL to*.93 17 ikilocycles .per Vsecond and by a 'process of division, employingcircuits of the type disclosed cti-pending' United States application,Serial FNKL 162,375 Ied v.Tully '21, 1947, United States iatent No.549,814 Igranted April (24, 15=1, `the 1-sinewaves :are yconvertedintosquare Waves hav `Ihretherecom-enceV 'frequency lof the station which`in `the particularfembodiment -is approximately "133 cyclesper second.'This recurrence frequency lis obtained, for example, 'by ra three'stage division lci i5.. 114 VAand lO. The `pulses Ygener-'ated yat the`stationipulse recurrence frequency, or 'PRF pulses `as they behenceforth termed, lare employed ztotriggerthe transmitter and also 4thetime base circuits L'I'Band of the two cathode ray Atubes. `Pls regardsthe transmitter Vpulse Vthis is `delayed `for a period of approximately"75 microlseconds since otherwise it will be invisible on `thecathoderay tube.. The .delayed pulses are `fed to the :modulator `unitMOD.

The calibrator :also provides Va series of pulses whichfare appliedtothe signal deiiection plates xoftbercathode rayl tubes for calibrationpurposes. 'Eline frequency of the pulses is such 'that verticalu.csleiiections of 'the spot occur at intervals along `time time basewhich represent a Vrai-ree of 1 mile. 'In addition `.theamplitude ofevery fth mile pulse iisiincreasedfso that the pulses provide, ineiiect, .a series `of 1` and `5 lmile calibration pulses.

ThePRF'pulses and th'ecalibration pulses `are edxto a marker `unit thechiel' function of to generate 25 .mile calibration pulses .item6/chexmile pulses, vand to pass` on lcalibration lpulses'.and PRF pulsestoother units. 'ln partie- `nlar' the marker unit Vfeeds negative-goingPRF :totrigger the two time base circuits 'LTB `and while negative-goingl and 5 mile ptilsesare lied Iseparately to the time base circuitFinallyand 25 mile pulses are fed to the mixer unit y Aspreviouslymentioned two cathode ray tubes "are employed one of which.. the longtime base tube, has a time base representing a rangeof 1650 miles Wlhilethe other, the magnied .time base tube, enables a small portion of thelong ltime base to be selected for presentation on 'an enlarged scaleonthis second tube. With maximum .magnication `2.1202 1/2v miles of thelong time base may be 4made'to cover the magnified time base, that is faVsoaleoi :about 6` inches :per milemay be obtained. Two Iadditionalscales onl the second tube `of 7 and 15 miles 'full deflection are alsopossible.

erence'to tle `i'blck"schematic"diagram 'shown 5 The following'indications vare ygiven `by the long time base trace: l

la.) the transmitter pulse (b) the return pulse at thc-:appropriaterange (c) a brilliance marker or strobe pulse which indicates theportionof the time base displayed the magnied time base tube (d) if desired 5and 25-.mle calibration pulses.

'The following indications are given by the magnied time base trace:

to.) ya blackoutpulse, known as the target marker,

whichV may be vaccurately positioned 4to represent the Vrange of `thetarget.

"du a brilliance pulse, 4known Vas Ythe walking strobe, which maybe.looked to the return pulse .and vtheir moves 'along the time 'base asthe :rangeof the aircraft from the station varies. (c) ifdesired1 1mileand 5 mile` calibration pulses.

As pointed out vpreviously the two time base circuits are triggered `bynegativegoing PRF pulses obtained from the marker unit and thenegativegoing 1 and 5 mile pulses obtained from 'the same unit are: tedto the magnified time base circuit for control-ling Vthe selection ofthe required portion of the long `time .baseior display on the magnifiedtime base tube. This operation is effected by .delaying .the .time `baseVtriggering pulse in 5 imile and l mile steps beh-ind the PRF pulses. An.indication is given on the long time base of the portion-.selected fordisplay on the magniiied :time base by an extra .brightening `pulse fedfrom the magnified to the long time base circuit.

The calibration pulses `and return signals are .fed tothe two time basecircuits from the 4mixer unit MIX. Positive going l and-5 mile pulsesare fed tothis unit direct from the calibrator unit While positive-going5 and .25 mile pulses are fed `to .the mixer unit from the marker unit.`Finally incoming signals `are. fed to the mir-rer Vfrom the receiverREC. Positive-going 5 and 25 mile pulses or the return signal Vare fedfrom the mixer through the long time base circuit yto the Y dedeflection.plates of the cathode ray tube (not shown) Further .positive-going 1and 5 mile pulses .and/or thereturn signalare fed `from Vthe mixerthrough themagnied time 4base circuit to the Y deection plates of thecathode ray tube (not shown).

The Walking .strobe unit WS is fed with a `neg- .ativeegoing returnsignal and also with a negativef-going trigger pulse fiornthe magnifiedtime .baseiunit-M'IB. The walking strobe unit has the following main.functions:

(a.) .it provides 4a Anegative-going pulse, the "walking strobe pulse,to the magnied time base unit: lthis pulse `can appear on any portion ofvtbe.magniiled `time base as intensity modulation. vThe `walking `strobeVpulse actually consi-sts `of two pulses, `one having twice the lengthof the other and having their front edges coincident, the .arrangementbeing such vthat the strobe can be locked .on to `any echo returned byan aircraft .which ,fisimoving `with any speed up to 600 miles `perfhour in any direction.

y(fb) it supplies `tvvo voltages to the pulse width 'control :unit PWCU,one oi which voltages is v linearly dependent upon the residual range ofVtlxle aircraft While the other is linearly dependent zuponthe radialvelocity: these voltages are employed to control the width modulation ofthe transmitted pulses to cause dot-dash signals to 7 be heard in theaircraft headphones for navigational purposes.

A common transmitting and receiving aerial A is employed and thetransmitterTR and modulator MOD may be of any suitable type to provide acarrier wave of 'approximately 10 cms Wavelength, pulse modulated at arepetition frequency of 133 cycles per second, the mean pulse widthbeing 3 micro-seconds while the minimum and maximum pulse Widths arerespectively 2 and 4 micro-seconds. Suitable switching arrangements areincorporated in the transmitter TR to prevent overloading of thereceiver RFC by transmitted pulses and to prevent the return signalspassing to the transmitter. Since the radio-frequency portions of theequipment form no part of the present invention, they will not bedescribed in detail. Further, the crystal-controlled oscillator TCCO,the calibrator CAL, the marker unit` MAR and the mixer unit MIX may beany conventional circuit arrangements which it will be unnecessary todescribe in detail.

The magnified time base unit MTB, the walking strobe unit WS and thepulse width control unit PWCU will, however, be described in detail withreference to Figs. 2, 3, 4 and 8. For this purpose reference will rst bemade to Fig. 2 which shows part of the circuit of the magnified timebase unit MTB.

This circuit produces a time base which 'gives rise to a trace on thescreen of the cathode 'ray tube representing 21/2, 7 o1' 15 miles atfull scale deection, the trace providing a magnified presentation of anydesired portion of the trace of the long time base tube. portion of thetrace of the long time base tube which is to be magnified is effected bydelaying the triggering of the magnified time base a variable time afterthe PRF pulse. The delay is not continuous but the adjustment is"effected in 5 mile steps as a coarse adjustment'and in 1 mile steps as ane adjustment after the PRF pulse.

The delay is provided by a valve V3 which is arranged in a circuit ofthe same general type as that described in co-pending United Statesapplication, Serial No. 762,375 filed July 21, 1947, United StatesPatent No. 2,549,874 granted April 24, 1951. The Valve is triggered bythe negativegoing PRF pulses derivedfrom the marker unit MAR (Fig. l).These pulses are fed from terminal T! (Fig. 2) through a differentiatingcircuit C5, RB to the cathode of the double diode V2. Since these pulseshave a sharp front edge and a sloping back edge, as shown in Fig. 5A,only the front edge will give a peak on differentiation. This peak willbe as shown in Fig. 5C, and will be transmitted through the diode V2 tothe grid of the delay valve V3. As a result of this the anode voltagebegins to fall as described in the above-mentioned specication, and thevoltage of the control grid and cathode also to fall due to feedbackfrom the anode circuit via the condenser C6 so that the section of diodeV2 connected to the control grid is cut 01T. A positive-going squareWaveform is produced on the screen grid of V3 and it is this waveformthat introduces the delay. As shown in co-pending United Statesapplication, Serial No. 762,375v led July 21, 1947, United States PatentNo. 2,549,874 granted April 24, 1951, the back edge of this waveformoccurs when the rising cathode voltage reaches the fixed suppressor gridvoltage. The anode is then cut off and the valve current all goes to thescreen with a consequent reduction in the voltage at the screen grid. Itwill therefore be seenthat if a The selection of the series ofnegative-going pulses are applied to the suppressor` grid, the back edgeof the screen grid Waveform can be synchronized with any desired one-ofthese pulses by suitably adjusting the maximum anode voltage on V3 bymeans of theA potentiometer R2. Actually negative-going 5mile pulses(Fig. 5B) derived from the calibrator unit CAL (Fig. 1) are fed to thesuppressor grid via terminalv T2 and condenser C1 so that theback edgeof the screen grid waveform, shown in Fig. 5D, may be synchronised witha selected 5mile pulse. In order togensure accurate synchronisation, thepotentiometer R6 is provided for varying the bias potential applied tothe suppressor grid.

Ihe positive-going screen grid waveform is differentiated by C8 and R14and the positivegoing peak is suppressed-by the other diode section ofV2 (Fig. 5E). The negative-going peak-is fed via terminal T4 to-thewalking strobe 'unit (Fig. 3) and via the condenser C9 to the cathode ofthe double diode V5 which is part of a delay circuit which operates in asimilai` manner to that just described. A positive-going square Waveform(Fig. 5F) is obtained from the screen grid of V6 and-the back edge ofthe waveform may again be locked to a selected one of a series ofnegative-going pulses appliedv to the suppressor grid but this time1-mile pulses (Fig. 5B) are applied to the suppressor grid via terminalT3 and condenser CI4. `The positive-going screen grid waveform isdiierentiated by C|5 and R28 and the positive-going peak is eliminatedby the diode V5 while the negative-going peak (Fig. 5G) is fed via thediode V1 to thetime base generating valves. The cathode of the diode V1has a positive bias applied to it to ensure that none ofthenegative-going l-mile pulses ybreak throughsto the subsequent stages. w

It has already been explained that the calibration unit CAL produces,under the control of the temperature controlled crystal oscillator TCCO,P. R. F. pulses and 5mile and l-mile calibration pulses. Thesecalibration pulses occur after the P. R. F. pulses to represent 5mileand 1mile steps of range respectively. Since the negativegoing portionof the screen 'grid voltage of the Valve V3 (the Waveform of which isshown in Figure 5D) can be synchronised with any selected 5mile pulse,say the mth pulse after the P. R. F. pulse, by means of thepotentiometer R2, and since the negative-going portion of the screengrid voltage of the valve V6 (the Waveform of which is shown in Figure5F) can be synchronised with any selected l-mile pulse occurring afterthe selected 5mile pulse, say the nth 1mile pulse after the mth 5milepulse, by means of the potentiometer RIG, the negative-going pulse(illustrated in Figure' 5G) fed to the time base generating valvesoccurs after the P. R. F. pulse at a time representing 5mg-1n miles ofrange `Where 'm and n are integers which have values .selected by theadjustment of the potentiometer R2 and the potentiometer Risrespectively. l

The circuit of the time base generating valves is not shown, as anycircuit suitable for generating a sawtooth waveform may be employed andsuch sawtooth generator circuits are Well known. The magnied time basecircuit MTB also includes amplifiers (not shown) for amplifying thereceived signal, the calibration pulses or both and feeding either, orboth of them to the Y plates of a cathode ray tube (not shown). Theamplifiers and cathode ray tube may be of any conventional and wellknown design.

lrangey of 197 miles.

' 9 Consideration will now be given tothe walkingstrobe unit shown inFigs. 3 and 4. This unit may` be conveniently divided into threesections:

(a.) Thevariable delay circuit (b) Thetarget marker delay circuit (c)The walking strobe circuit The variable delay circuit is triggered bythe negative-going peak obtained after differentialtion of the output ofV3 (Fig. 2) or the magnified time base unit. This peak, it will beremembered, is locked'to a selected one of the -fmile pulses. Thevariable delay circuit produces a positivegoing pulse which is delayedbeh-ind the triggering pulse `by an amount which may be varied between15 and 90 microseconds. This positive pulses employed trigger the targetmarker delay circuit and the walking strobe circuit.

The target marker delay circuit is similar to the variable` delaycircuit but theA delay introduced thereby is normally about Simi-les andonly a small adjustment of approximately il mile may be effected. Thecircuit produces a. 1 microsecon-d negative-going pulse which appears onthe magnied time base trace as the target' range marker.

Ihewalking strobe circuit produces a negatiyegoing pulse which is fed tothe magnified time base lunit andV appearsat any position on the timebase as intensity modulation, Hereinafter the appearance of this pu-lseon the time base will be termed the walking strobe." The walking strobeis capable-of locking to any echo returnedv by an aircraft whichi-smoving with any speed upto 600 milles an hour in any-direction. Thereturn signal from the aircraft is fed to the walking strobe circuit'and to lock the walking strobe tol any echo", an autoemanual switch isprovided, with a controlI for the manual' adjustment of the position ofthe walking strobe. With theY switch in the manual position, the walkingstrobe is set to a suitable position on a certain part of the tube knownas the active region whereupon the switch i'sthrown to the auto-positionand the walking strobe locks to the echo signal as the signal overlapstherewith. Further in' the walkingstrobe' circuit two voltages are1produced, one of' which is linearly dependent on the residual range oi?the aircraft While the other is linearly dependent upon thel radialvelocity ofthe aircraft. These voltages arefed to the pulse widthcontrol unit.

The relation between the various delays introduced by theY differentcircuits will be" better understood from a consideration of Fig. 6. Fig.represents the long time'base trace and shows `the S-mile delay betweenthe PRF pulse andY the zero position on the trace, at which zeroposition the transmitter is pulsed. The long' time" base trace isdivided into 5` and 25-n1ile sections. Fig. 613iz shows the transmitterpulse and the target Fig. 6U shows a possible positionl for the 5-mileselector" pulse in which the pu-lslerisv locked? to the 38th five milepulse'i. e; at 185l miles. Fig. 6D shows theportion of the long timebase trace betweeny FZ5 andv 210L miles on an` enlarged scale. Fig. 6E?shows the 5`mile selector pulse lockedl at 185 miles and the 1-mileselector pulse' looked at 189 miles. The l--inile selector pulsetriggers the magnied time baseand'f the trace is indicated as covering arange of miles. New the target-marker has to be positioned at 1197miles; andi the 12--1nile active region extends for 6 miles each sidelof the targetlmarler i; e. from 191. 1:0220?" miles.` The delayintroduced bythe variabler delay circuit; must therefore be set for1end!` of' the` grid waveformr (Fig,4 5P) 10 the difference between and191 miles i.- e. 6 miles` and the target marker will then be inthecorrect position.

Referring now to` Fig. 3, the variable delay circuit consists of thevalves VIM, V192, Vll, VIM and` V195. The negative-going pulse from theniagniw` time base unit (Fig. 5E) is fed via terminal T4 to thedifferentiating circuit CIDI, R493: and the output (Fig. 5H) is fed tothe cathode of` the diode V102; lThe 5-mile pulses which are appliedtothe-suppressor grid of the valve VfV (Figure 2) may break through thevalve to produce negative voltage pulses on the screen gridof the valveV3 (Figure 2) and at the terminal T4. In order to prevent thisbrealrethrough from affecting the variable delay circuit, the diode V102has a positive bias of. approximately 6 volts appiied to its cathode.The negative-going pulses from the anode of Viti?l are then applied tothe control grid of Vidi which is normally cond-ucting, A positive-goingpulse (Fig. 5J) is thus obtained at the anode of Viti and is fed to thesuppressor grid of VIG3 w ich forms part of a delay circuit of the typedisclosed in United States application, Serial No. '762,375 filed July2l, 1947, lllriitedv States Patent No'. 2,549,874 granted April'zi,1951. positive-going square wave (Fie 5K) will be obtained at the screengrid` of Vlthe width of` which isvariable beinyeenl5' and. 80microseconds by adjustment of thefpotentiometer R141. The screenwave-form is-now differentiated by C-l, RH? and RHS (Fig. 5L) thepositive peaks being removed by thefother halt of the. diode Vllll. Thenegative peaks. are applied. to the grid of V605, which is normallyconducting. and produce at the anode a positive-going substantiallysquare Waveform (FigM 5M). This waveformwi-lllbe delayed-behind theinput triggering pulse by an amount dependent upon the active regiondelay control i. e. the potentiometer R141.

The output fromVl Q5 isfed viaI Cl H Rl 49-and R450 tothe suppressorgrid of Vl-llt Whichtogether with Vid?, Vlfand V me ferm the targetmarker delay The valve Viet forms part oiA ai delay circuit of the typedescribed in; copendi-ng United. States application, Serial No. '162,375filed July 21,V 1947, United States Patent No.` 2,549,874 granted April24, 1951,. In this casse the potentialto which the anode of Vlot returnsis. iixed4 and theV potential to which the grid leak R431` is connected;is variable by means of the potentiometer RIM; The wid-th of the screen.vavcform obtained with thisY circuit corresponds approximately to 6miles and variation of RI-34 enables the width to be varied by 1 mile.VThe screen waveform from Vidi; (Fig. 5N) is fed to the control grid; ofV598', the. bias potential of which` is normally 10. volts so that thevalve iscut oi'. The screen waveform is positive-going and thus causesthe valve to conduct and a negative-going waveform. is obtained fromVthe anode. The anode load. of V`l0`8 includes a. network` having a delayof. about 1/2 microsecond. This network is short-circuited at the farend and the resultant waveform at the anode will thus consist ofV a 1`microsecond negative-going pulse at the-beginningoflthe positive-goinggrid waveform andl a- 1 microsecond. positive-goingpulse at the l A YThe output fromVlB 4is fed. via Cl I=5 and` RMS to the control grid `of-Vkt which isnormally cut offv so that the 1. microsecond positive-goingpulse is alone effective, going al microsecond4 negative-goingpulse atthe anode of VIABS (-lig.` 513),` This pulse is signal and each strobepulse.

inafter termed strobe l has a width of 1%,

jfed via C! l and terminal T5 to the grid of the cathode ray tube in themagnified time base unit 'RM1 (Fig. 4) which determines the extent ofthe delay introduced by the variable delay circuit before the targetmarker delay circuit is triggered. These two controls functionrespectively as the -coarse and fine controls for setting the targetmarker (see Fig. 6).

The output of the variable delay unit is also 'fed via C! I! and Rl23 toterminal T0 leading to the circuit of Fig. 4. This circuit may for con'-venience be divided into two parts; rst that part,

'consisting of V205, V200, V201 and V203, which produces the walkingstrobe pulse and second that part, consisting of the remainder of thecirlcuit, which provides the range and velocity voltages and whichcauses the walking strobe to lock on to an echo.

The walking strobe pulse is generated by the -valves V205, V200, andV207, V200 being triggered -by the positive-going waveform applied toter- 4minal TB from Fig. 3.

In the present arrangement, the walking strobe consists of twooverlapping pulses shown in Figs. 7B and C. These two strobe pulses arephase-reversed and fed as negative-going pulses to the cathodes of two'strobed diodes V2l2 and V2l5, the echo signals, 'which arepositive-going, being fed via terminal T9 to the anodes of the twodiodes in parallel. The current ow through the diodes will thus dependon the degree of overlap between the echo One pulse, heremicrosecondswhile the other pulse, hereinafter termed strobe 2 has a width of 21/2microsecoverlap of the signal with strobe 2 tends to cause 'the pulsesto move to the right. The circuit is so arranged that a balance isachieved when the centre of the signal is at the centre of strobe 2.

' The overlap of the signal with strobe l is then one f half the overlapwith strobe 2 and if i1 is the current due to strobe l and i2 is thecurrent due to `strobe 2, then 11:1/222. The current due to strobe 2 ispassed through a phase reversing L valve V216 which reduces the currentto half its vvalue and the two currents are then fed to the grid circuitof the valve V20I.

In the particular case under consideration the sum of the currents Aiszero and hence there will be no current ow into the condenser C20! whichcouples' the anode of the valve to the grid. The anode voltage thus'remains steady, and the strobes remain stal tionary.

'Ihe voltage variations of the anode of the valve V20! control, in amanner to be explained -in detail later, the movement of the walkingstrobe along the magnied time base. The arrangement is such that foreach value of the anode voltage there will be a denite velocity of f thewalking strobe along the magnied time base. The anode voltage of theValve V20! will thus be termed the velocity voltage and the valve fitself will be termed the velocity Valve.

Now flow of 'current into the condenser C20! will cause the anodevoltage of the velocity valve V20! to fall linearly while flow ofcurrent out of the condenser will cause the anode voltage to rise.Suppose that the echo signal is completely within strobe Then i1=i2 andthe current due to strobe 2 is phase reversed and halved by the valveV2I6 so thatthe resultant current is 1/21'2, and ilows out of thecondenser and the velocity voltage rises linearly. The circuit is soarranged that, as the velocity voltage rises, the strobes move to theleft to bring the echo signal towards the centre of strobe 2. Similarlyif the signal is entirely within strobe 2, i1=0 and the current due tostrobe 2 flows into the condenser C20! and the velocity voltage fallslinearly. The strobes are then caused to move to the right. With nosignal i1=i2=0 and the velocity voltage remains constant. The velocityvoltage is integrated by V202 to give the residual range voltage whichis fed to the `cathode of V200, thereby controlling the timing of thewalking strobe pulses as described in detail subsequently.

Assume that the echo signal is over-lapping one or both the strobepulses. The circuit is so arranged that, under this condition, thecurrent flow through the valve V2! 'l is suicient to maintain relay MRoperated. With the relay operated, condenser C203 is charged to avoltage equal to the mean value of the velocity voltage. If now .thesignal fades, the current through V2I`! falls linearly with a time delaysuch that the rela-y MR releases if the fade has a duration of greaterthan approximately 1/th second. When the relay releases the grid of V20!is disconnected and C203 in series with R200 is applied between theanode and grid of V20! The condenser then maintains the anode of thevelocity valve at a steady value equal to the mean velocity voltagebefore the fade occurred.

A detailed description will now be given of the circuit, commencing withthe generation of the two walking strobe pulses. As previouslymentioned, a positive-going trigger pulse is fed through terminal T0 tothe suppressor grid of V205. The valve V205 forms part of a delaycircuit of the type described in co-pending United States Application,Serial No. 762,375 led July 21, 194'7, United States Patent No.2,549,874

granted April 24, 1951, and produces a positivegoing wave-form (Fig. 7A)at the screen, the width of the waveform depending upon the volt- .ageto which the anode is allowed to rise as determined by the cathodepotential of the diode V204. The screen Waveform is fed to the controlgrid of V201 which is normally biassed to cut oil, the positive-goinginput waveform thus giving rise to a drop in anode voltage, producing(as shown in Fig. 7B) a negative-going anode waveform. The anode load ofV20! includes a delay network DN- one end of v/hichis short-circuitedwhile the other end is terminated by a revsistance R239 equivalent tothe characteristic impedance of the network. The network is so arrangedthat the delays introduced by the section cb, bc and cd are each equalto 5A; microsecond. vI-Ience at the point a, a negative-going voltagepulse waveform (Fig. 7B) will be produced dethe waveform at b to travelto d and back from d to a less the time taken for the waveform at b totravel to a i. e. (5-l)5/;=4 %=21/2 microsec- Similarly at c anegativefg'oing voltage -13 pulsewavefonm (Fig. 7C) `will be: produceddelayed bzwfa similar amountand having a width of X1%irnicrosecond.Further the negativegoing edge of the Waveform from the screenI @EV-M55will cause thegeneration of 'two similar positive-going pulses by thedelay network "vi-he output: from the points a and `cottlfre network isapplied -via dfill,A 'R262 and C215, R252 `respectively to the controlgridso'f Wigs-f and Willi.- -Both these valvesare normally biassed tecutfoif so that only the positive-goingfpulses generated by the .networkare effective and itwill be seen, therefore, that the timing ciA thesetwo 4pulses i. e. strobe iii andstrobe 2 willv bedependent on thewidthoiithe waveform obtained from the screen of "tutti te sayenunscathed@potential or W204i The neefativweoinganode waveforms ,of Vlf and V213are fed to the cathode ci V212 and V215 respectively; the diodesvrhi andV2M being provided to prevent the anodes--of' Val-c and'V2-t3 fallingbelowl earthpotential.- in addition; theV screen waveform fromv Vtiwhich wilt-be negative-going isfed to ther-controllanti orm V2'llil-`which phase reverses it, fthe positive` going anode waveform being fedvia '1i-'lc tothe `ca'tl'iode of the niagnied cathode ray. tube tobrighten the trace in the region ofthe walking strobepul'se; s

`The echo signals, after amplification in circuits'whicharefnot shownin' the draw-ing, are fed via tern'iinal T9 and condenser C22-'litt'oflthe "cathode follower valve V209; r)This valve is na IWB beamtetrode` andV provides `apositive-going -signal"'across the cathoderesistance R25@ which i is fed in parallelfto the anodes of the twodiodes vri-r and vz-i-e.- The valves vri-s veis are both Mazda valves,'type Simi-l while the four diodes V2`H V212, V2M and V215 are allMuilard valves, type EAEO.

Thecatnodes lof the-diodes V2 l2 and V215 are i normally maintainedat+150vol-ts and this po tential is reduced'to earth by the Vrespectivestrobe pulses. The anode 'or V212 is connected via R25? and R258 tothecontrol grid of thelvelocityvalve Vztwhilethe anodeof V2i5'is conuIect'ed via R259 "and Rifiuto' the cOntroiy gif-id of the phaselreversing valve V2 it. Both "anodes are thusV normally4 atVapproximately earth po tential 'while' the v'incidence or a signal pulseraised theirpotential to a maximum of '110 volts. 'lhus when there isno'overlap between the echo signal and strobe pulses, bothdiodes will becut off. If, however, a signal overlaps with strobe 'Ifthe' diode AV212conducts and the condenser C216 receives a' charge.' This charge leaksaway through `the alter circuit consisting of R251, C1211 and R258 .andconstitutes a current now outiof condenser 'C20 l. Similarly, the diodeV2! 5 conducts and the condenser C222 receives a :charge when the echosignal overlaps strobe 2.

charge leaks away through the filter crcuit.consistingoiRESg C223 andR21!! and constitutes a current il'ow out of condenser C224. :ThecondenserCZli is connected between the 4`anodetand grid'of thephase-reversing valve V2 IS which is provided With negative feedback dueto `the connection of resistances RZH and R2l'5 between'the controlgridand anode. The cir- `cuivisso arranged that, when there is no overlaptbetween strobe and signal pulses, there is zero current flow throughthe grid leak R211. Theivoltage at the control .gridis then -2 voltsfandwhence the lvoltage of. the junction of. R215 and; will `also be .-2volts. 'TakingA into consideration :the 4current flow through B216,

R211,v R218, R2192 and R28D, it can be shown that the .anode voltage `otVitt at this time iS 35 volts. Now suppose that a strobe current of ismicroamps ilows out of the condenser S224; The anode voltage ot V2M'begins to rise' andl owing Vto the feedback .through R275,V and R211,the `gridY voltage also incr-eases and `gri-d current begins to flow.Eventually the state is. reached at which the currsntiiow AthroughRloand the currentfflow through R272, i. e. the current new throughR21-l, balance andJ the potential at the unction of Riiiil andl R212becomes -steouiy1 '.No further current ilows out of the condenser C22-5and hence the anode voltage `remains steady. The junction of RzifandH2325 must now be at a voltage given by lOi2-2 and hence the anodevoltage of- V2hi will be givenby 1Oiz-12+`1/2 (sumrof the currentsinmicroamps throughtRl l R286, R211, Pta-fm, R2 is and R289.)

Itvwill be explained subsequently that thergrd potential of Viii isapproximately -5"volts so that the current through VR2tl flowing intothe condenserCEl'azl `willbe given by Y .K(102-2+5D /10 mierda-rupswhich is .approniinately Kriz 'where K. depends on thesetting `of thepotentiometer R218. The latter is set so that, asexplained previously.,the current flowing into the condenser is 'is i. e; K: /lg.

It will thus be seen thaty the. current ilow due to overlap between theecho signal and strobe l passes out of the condenser C201 while that dueto overlap between the echo signal and strobe 2 passes into thecondenser. The condenser CZBi is lconnected between-the anode andcontrol grid of thevalve V20! which is a'Mazda type SP41. `Thearrangement is such that, with the. relay MR' operated, a D. C. biaswithin the grid base of the valve is applied to the controlrgrid. Thisbias maintains the anode. voltage vsteady while a ow of current into thecondenser CZDI causes the anode voltage to fall linearly whilel a i-lowof current out of the condensercauses the anode voltageto riselinearly.As previously mentioned,

for leach value of theanode voltage of` the valve 4'setto position 3when 165 volts is fedover'bank vI whereupon the potentiometer R213 is`adjusted until there isno movement ofthe walking strobe along the timebase. It should be explainedfat thispoint that with the switch Si inposition 4, a voltage from H2M?, variable from about "70 to 250 voltsisfed to the control grid of V2M forA test purposes, while with the switchSl in position 2, the velocity voltage from the anode of V201 is fedtothe control grid of the valve V202 assuming the switch S2y to be inposition 4. The valve V262 is arranged by virtue of the ,condenser C206to integrate the velocity voltage iedto theA control grid to providevoltage proportixnialA lto` theresidual range. The operation of thistype of circuitihsnwell known and depends upon the current iio'wvthrough the resistance R234 which causes current to flow into or out ofthe condenser C206 which produces a gradual change of potential(increasing or decreasing) at the anode of V202. This change ofpotential .which is proportional to the residual range is fed via bankof the switch S4 in position 3 to the cathode of the diode V204 and, aspreviously explained, the changes in cathode potential control thetiming of the walking strobe and hence the position the strobe occupieson the magnied time base. The circuit components are so chosen that arange voltage of 165 corresponds to the target range. With this denitionit will be understood thatwhen the walking strobe circuit is fed with165 volts, the target marker delay circuit must be adjusted so that itproduces a target marker pulse which is also at the target range. ThisIadjustment is effected during the initial setting upfof the equipmentand for this purpose the switch S4 is set to position 2 so that 165volts is applied to the cathode of V204 and the resistance R!34 (Fig. 3)is adjusted until the target marker pulse sits at the centre of thewalking strobe as observed on the magnified time base. Thereafter whenthe target marker pulse is set to a particular target range, the centreof strobe 2 of the walking strobe will coincide with the target rangewhen the range voltage amounts to 165 volts.

In operation, the walking strobe pulses are manually adjusted to asuitable position in the active region whereupon the auto/manual switchS2 is thrown to the autov position and the walking strobe locks totheecho signal as this signal y type SP41 and with the component valuesshown this value of anode voltage will be obtained with la D. C. bias of-5 Volts. Under these conditions there will be a current flow of 1.9microamps `through R! and this current would lnormally iiow into thecondenser C20! producing a fall in 4the anode voltage of V20I. In orderto prevent this, the resistance nei-.work R202, R203 and R2 l1 isprovided and the tapping of R202 is adjusted until a current of 1.9microamps flows through R2 I! to balance the current flow through R20When the signal voltage overtakes the walking strobe it will initiallybe equally within both strobes. The currents through the diodes V2!2 andV2 5 will then be equal but the current flowing out of C20! due to thestrobe I current will be twice the current flowing into C20! due to thestrobe 2 current. There is thus a net current flowing out of C20! andthe anode voltage of V20! will rise to its maximum value. As theaircraft approaches the point at which the exponential path isasymptotic to the circular path, the net current flowing out of C20!gradually decreases and the anode voltage of V20! will gradually falluntil when the aircraft is over the target, which is arranged to be atthe point where the circular and exponential paths merge, the anodevoltage is again 165.

Finally it should be explained that the velocity and range voltages arefed to the pulse width control unit (Fig. 8) viaterminals `T! and T8respectively.

As previously mentioned, the relay MR is operated during the time thatthe walking strobe is locked to an echo signal and releases if thesignal fades during this time. Prior to locking, the relay wouldnormally be released but arrangements -are provided for preventing therelay exerting any control during this period. The relay is located inthe anode circuit of the valve V2l1, the anode of which is coupled tothe control grid by the condenser C225. The valve is arMazda type SP41.It will be noted that the grid side of the condenser is connectedthrough R285 to the junction of R284 and R286 which forms apotentiometer across the 250 volt'supplained.

ply. The junction of R284 and R288 is thus at a voltage of and, sincethe control grid is at a potential of approximately -2 volts, there iscurrent flow of 25 microamps into the condenser C225. There is however,a current iiow out of the condenser through R283 and this current flowdepends on the anode voltage of V! 6., The circuit is arranged so thatwhen echo signals above a predetermined level lare being received, thereis a resultant current flow into the condenser and the anode voltage ofV2!! falls, the current flow in the anode circuit being sufficient tomain-y tain the relay operated. If the strength of the received signalsis at the predetermined level, the currents into and out of thecondenser balance and the anode voltage remains stationary with therelay just on the point of releasing. If the strength of the receivedsignals falls below the predetermined level, there is a resultantcurrent flow out of the condenser, the anode voltage rises and the anodecurrent falls below the value required to maintain the relay operated.The relay thus releases to effect certain circuit changes which will bedescribed subsequently.

It was pointed out previously that, with no current flow through thediode V2!5 i. e. no overlap between strobe and echo signal, the voltageat the anode of V2I6 is approximately 35. The junction of R28! and R282is held at -30 volts by the diode V2|8 at this time so thatv a currentof approximately 50 microamps flows out of the condenser C225 throughR283. Taking into account the current flow through R285 it will be seenthat a resultant current of 25 microainps flows out of the condenserC225. The anode voltage of V2l7 thus rises and they relay is in itsde-energised condition as previously ex- If now the diode V2!5 passescurrent i. e. there is overlap between the signal and strobe pulse, theanode voltage of V2M; rises as pre viously explained. This rise in anodevoltage will be without eiect until the current flow through R28! andR282 changes suiiiciently to make the cathode of V2i8 positive withrespect to the anode i. e. the junction of R28! and R282 is morepositive than -30 volts. The diode V2 I8 then ceases to conduct and thecurrent ow out of the condenser C225 through R283 begins yto decrease.The circuit is so arranged that when the anode voltage of V2!6 reaches112, the june-.- tion of R28! and R282 is at -171/2 volts when thecurrent iiow into C225 balances the current flow out of C225 and theanode voltage of V2!'! remains stationary with the relay just about tooperate. Further increase in the anode voltage of V2I6 causes thecurrent flow through R283 to decrease below 25 microamps so that thereis 17 now a resultant current flowing into the condenser and the anodevoltage of V211 falls and the relay operates.

If the strength of the echo signal falls below the predetermined level,the junction of R281 and R2E32 will return to -30 volts and the anodevoltage of V211 will rise with a time constant which depends on thevalues of C225, R283 and R285. With the values shown in the diagram, thetime constant is such that the relay MR will release if the fade lastsfor longer than 1/gth second. If the signal reappears during thisinterval, however, relay lvl'lt remains operated, the energising of therelay being more rapid than the de-energising owing to the provision ofthe diode V218 which limits the current which can flow out of thecondenser C225. It will be understood that, with an echo signal pulsewidth of 1 microsecond, the current flow through the diode V215 will besubstantially proportional to the strength of the signal and hence theanode voltage of V216 is also proportional to the strength of thesignal. Variation of the level atA which the relay MR should release maybe controlled by varying the amplification of the signal amplifyingstages (not shown).

VWhen relay MR is operated, i. e. there is overlap between the echosignal and strobe 2, contact mrd connects a source of volts to the lowerplate of C203 while the upper plate is connected through R208 andcontact 'mr3` to the anode of the velocity valve V201 via the smoothingnetwork C202, R201. The condenser C203 is thus charged to a voltageequal to the mean velocity voltage plus the D. C. voltage applied to mrd(-5 volts). Further contact'mrl connects the 'condenser C201 between theanode and grid of V2M. When relay MR. releases the condenser C29 isdisconnected from the control grid of V201 and the condenser C203 isconnected between the anode and control grid via m13, R208 and mrl. Thecondenser then maintains the anode of V201 at a steady voltage and,since the grid voltage of V201 is approximately -5 volts at this time,the anode remains at a voltage equivalent to the mean Velocity voltagebefore the fade took place. Further, at contacts mrd, the lamp LA islighted and, at contacts m12, the resistance R291 is connected acrossthe 350 volt supply so that the consumption is independent Iof whetherthe relay is energised or deenergised.

As previously mentioned, the relay must not be allowed to de-energisewhile an echo signal is moving along the trace prior to locking to thewalking strobe. For this purpose the switch S5 is provided and, in theposition shown, bank 1 connects earth to the anode of V211 so that therelay is energised as long as the switch is in this position.

It' will be understood that the charge on condenser C203 will eventuallyleak away so that the voltage between the anode and grid will fall. Thecircuit constants have been soy selected, however, that the anode/gridvoltage willremain substantially constant for fades lasting up to 1minute. After this time the anode voltage will gradually fall so thatthe movement of the walking strobe along the time-base trace slows downand eventually remains stationary. If the signal reappears after thisinterval the walking strobe will lhave to be manually adjusted t0 theappropriate part of the active region where it will remain until thesignal again locks to the strobe. Ifthe fade lasts for lessV than v1minute, how- 18 l ever, the echo signal should reappear on the time-basetrace in such a position it will be within the region controlled by thewalking strobe. Automatic control as described above will then come intooperation immediately. `It will be understood that it is only after afade that the echo signal is ever within strobe 2 only. Normally whenthe echo signal locks to the walking strobe, the echo will be at theleft hand edge of both strobes and will gradually progress to the centreof strobe 2 in which position-it overlaps also with Strobel.

Consideration will now be given to the operation of the pulse widthcontrol circuit which is shown in detail in Fig. 8. As previouslyexplained communication between either ground station and the aircraftis effected by variation in the pulse width of the transmitted signals;The normal pulse width is 3 microseconds and provision is made forvarying this width between the limits of 2 and 4 microseconds. Filtercircuits included in the aircraft equipment convert the pulse widthmodulation into amplitude modu` lation, the arrangements being such thata suc# cession of 2 miscrosecond pulses produce an inaudible amplitudewhile a succession of 4 mi crosecond pulses produce an audible amplitudeinA the headphones.

Intermediate values of pulse width produce in-i termediate soundamplitudes.

rlhe 3 microsecond pulses will be transmitted as long as the aircraft ison the correct track, the pilot then hearing asteady note in the head-iphones. When, however, the aircraft is on track the station sends out aseries of pulses of widths alternately (i3-D) microseconds and (S-I-D)microseconds, the quantity D being a function of the residual range andthe rate of change of residual range. The quantity 1WD, where D ismeasured in microseconds, is dened as the percentage modulation and themaximum modulation used in practice is per cent. This, therefore,corresponds to alternating 2 microsecond and 4 microsecond pulses. Thefrequency at which the two widths alternate is 2 per second and theratio of the time during which (3-1-D) micrcsecond pulses aretransmitted to the period of time during which (3-D) microsecond pulses'are transmitted is about 1 to 6 for dot trans` mission and 6 to 1 fordash transmission. Dash transmission indicates to the pilot that he isheading away from the ground station while dots indicate that he isheading towards the ground station. Zero modulation produces a steadynote and indicates that the aircraft is ying along the correct track.

The pulse width control unit receives the range and velocity voltagesfrom the walking strobe unit and controls the dot-dash width modulationof the transmission to keep the aircraft on track, the pulse width ofthe transmitted signals being controlled by a D. C. potential fed fromthe pulse width control unit to the modulator MOD.

The anodes of the two valves V301 and V302. are connected together and.to the resistance chain R309, R316 and R311, the valves acting ascathode followers with a link R314 between their cathodes. The rangevoltage developed in the walking strobe circuit is fed via terminal T3and resistances R318 and.R319 to the control grid of the valve V302while the velocity voltage is fed via terminal T1 and a chain ofresistance R301 to R305 to the control grid of V301. The

potential of volts, corresponding to zero veacsaare said contact to saidload circuit, means responsive to the strength of the incoming signaland coupled to said relay, said relay being controlled in accordancewith the strength of the incoming signal by said last named means tovary the positioning of said contact so as to feed said control voltageto said condenser if the strength of the input signals is above aminimum value and to connect said condenser to said load circuit if thestrength of the input signals is below said minimum Value.

5. Circuit arrangements according to claim 4 wherein said meansresponsive to the strength of the incoming signal comprises a thermionicvalve having said relay in the anode circuit and a condenserV connectedbetween the anode and control grid, means for causing current to flowinto or out of said condenser according to the strength of the inputsignal to vary the charge upon said condenser and in turn to vary thecurrent flowing in the anode circuit of said thermionic valve, saidvarying anode current energizing the relay when the strength of theinput signal is above the minimum level and de-energizing the relay whenthe input signal is below said level.

6. The combination set forth in claim 4, said source of locallygenerated recurrent signals having an output with two recurrentsubstantially rectangular wave forms with coincident leading edges.

'7. The combination set forth in claim 4, said source of locallygenerated recurrent signals having an output with two recurrentsubstantially rectangular wave forms with coincident trailing edges.

8. Circuit arrangements comprising a source of recurrent input signals,means for generating local signals consisting of two recurrentsubstantially rectangular waveforms having coincident edges anddifferent Widths, the width of the second of said waveforms beinggreater than the width of the rst of said waveforms by an amount inexcess of the width of an input signal, means coupled to said inputsignals and to said local signals for obtaining a rst current inproportion to the duration of co-existence of the input signals and thefirst of said waveforms and means coupled to said input signals and tosaid local signals for obtaining a second current in proportion to halfof the duration of co-existence f the input signals and the second ofsaid waveforms, means responsive to said iirst and second currentsincluding means for deriving a control voltage proportional in magnitudeand sense to the dii-ference between said first and second currents, aload circuit and a condenser, means coupling said control voltage, loadcircuit, and condenser, and feeding said control voltage to said loadcircuit and to said condenser to charge said condenser to a voltageequivalent to the mean value of the control voltage and switching meanscoupled to said source of recurrent input signals, said switching meansincluding means operative in response to a drop in signal amplitudebelow a predetermined minimum value to cause said condenser to dischargethrough said load so that a substantially constant control voltage equalto the mean value is obtained.

9. Circuit arrangements for the generation of a control voltagecomprising an input adapted to receive a plurality of input signals,square wave generator means, said square wave generator means includingmeans to produce two recurrent square waves having coincident edges anddifferent wid'ths, said square wave generator means having an outputoperatively connected thereto and including means feeding said recurrentsquare waves to said output, means coupling said input to said output,first comparison means responsive to the period of cci-existence of saidinput signals with one of said recurrent square waves, said firstcomparison means including means producing a first current bearing aconstant ratio to said period of co-existence, second comparison meansresponsive to the period of coexistence of said input signals with theother of said recurrent square waves, said second comparison meansincluding means producing a second current bearing a fractional part ofsaid ratio to said last named period of co-existence, voltage outputmeans, means coupling said first comparison means and Vsaid secondcomparison Y means to said voltage output means, said voltage outputmeans including means responsive to the sense and magnitude of saidfirst current and said second current for producing a control voltagewhich varies with variations in the magnitudes and senses of said firstandsecond currents.

10. Circuit arrangements as claimed in claim 9 wherein said square wavegenerator means includes a valve circuit for the production of aplurality of substantially rectangular waveforms, delay means, meansconnecting said delay means to said valve circuit for feeding saidplurality of substantially rectangular waveforms to said delay means,said delay means including two terminals electrically spaced from oneanother, said delay means being so constructed that said two recurrentsquare waves having coincident leading edges and different Widths appearrespectively at said two terminals.

11. Circuit arrangements as claimed in claim 9 wherein said voltageoutput means includes a valve having an anode and a control grid, acapacitor connected between said anode and said control grid and meanscoupling said first current and said second current to said capacitor tovary the charge upon said capacitor in accordance with the sense andmagnitude of said first current and of said second current.

12. A pulse echo circuit comprising a source of recurrent input signals,a source of locally generated recurrent signals, means responsive tosaid input and locally generated recurrent signals for deriving acontrol voltage substantially proportional to the duration ofcoexistence of said input and locally generated recurrent signals withone another, a load circuit and a condenser, circuit means to feed saidcontrol voltage to said load circuit and to said condenser to chargesaid condenser to a voltage equivalent to the mean value of said controlvoltage, and switching means coupled to said source of recurrent inputsignals and operative at a selected minimum value of said input signalto cause said condenser to discharge slowly into said load circuit, saidswitching means comprising a relay having a pair of contacts in thecircuit of said condenser, means coupling one of said contacts to saidcontrol voltage and further means coupling the other of said contacts tosaid load circuit, said relay being controlled by said input signals toso position said contacts as to couple said condenser to said controlvoltage at values of input signal above said selected minimum.

13. The system of claim 12 in which said relay is controlled by saidinput signals to so vposition said contacts as to couple said condenserto said load circuit at values of incoming signal below a selected 14.The system of claim 13 including an elec- .messie-19 23 24 trpnftube,said relay, beg ,connectedin the plate

